1. Field of the Invention
This invention relates generally to the encoding of control information to a substantially continuous web of materials and manufacturing methods and apparatus utilizing such encoded webs. More particularly, the invention relates to a system which is especially adapted for use with webs for use in packaging and other applications.
2. Prior Art
Continuous plastic webs are manufactured for many purposes. As an example, chains of interconnected open bags such as those described and claimed in U.S. Pat. No. 3,254,828 to Hershey Lerner have been sold successfully under the trademark AUTOBAG. As another example, plastic mailing envelopes made from webs such as those disclosed in U.S. Pat. No. 3,641,733 to Hershey Lerner have been sold successfully under the trademark ZIP-VELOPE. In the manufacture of both the AUTOBAG and ZIP-VELOPE products, a web of plastic is first printed to provide identifying information and an attractive appearance. In subsequent manufacturing operation transverse seals are formed between two layers of the web. In the case of commercially produced AUTOBAG products, spaced transverse perforations are formed to provide lines of weakness for separation of the bags from the web.
Chains of interconnected bottle labels have also been produced in quantity. A label chain is in the form of a plastic tube which is perforated between each adjacent pair of labels to allow each label to be separated from the chain and placed around a blow-molded plastic, or similar, bottle.
In the manufacture of webs of material such as chains of bags or labels and strips of envelopes, it is important that manufacturing operations be accurately located along the web. As an example, the transverse seals obviously should be between adjacent bags or envelopes and not in central portions of them. Accordingly, it is important to accurately register the web with work stations on the machine performing operations on the web.
While there is reasonable latitude or tolerance in the location of any given operation on a web, there is a cumulative error problem which must be considered. For example, if each seal in an AUTOBAG web is mislocated by 0.001 of an inch so that each bag being formed is longer than it should be by that amount, and this error is allowed to repeat each time a bag is formed without error corrections, by the time the 1000th bag is formed the seal will be misregistered by one inch. Obviously, if one is transforming a printed web into a chain of bags, a strip of envelopes, or a string of labels such cumulative error cannot be tolerated.
The cumulative error problem is exacerbated when the web is plastic because plastics tend to stretch. Since it is virtually impossible to maintain constant web tension during printing and other manufacturing operations, stretching not only occurs but it occurs unevenly.
Because of the cumulative error problem, it is customary to repeatedly register the web with stations where manufacturing operations are to be performed. One known technique is to provide clear spaces in a web between the repetitive printed indicia which spaces function as "windows". A registration mark of some type is imprinted in the window. An optical detector is positioned to cyclically view the web. If the equipment is adjusted and functioning properly, each viewing of a cycle is concurrent with the passage of one of the windows past the detector. The detector senses the registration mark and causes the manufacturing operation to occur at a time coordinated with this sensing.
When printed decorative and informative indicia on the web is passing the detector, the detector is "blinded" so that it will not see and be confused by the imprinted indicia. Expressed another way, a detector should be turned off as decorative and informative indicia passes it and turned on when the detector is registered with a window.
A major problem with a cyclical detector which is "blinded in each cycle is that if the web is out of registration so that the detector is operative when the decorative and informative indicia are under the detector, the detector emits erroneous signals and the machine will produce scrap. Thus, machine set up, and the restoration of appropriate registration of the machine gets out of synchronism, is time-consuming and difficult.
The effectiveness of traditional registration marks for controlling operations even on essentially a clear web; that is a web which is not printed except for the visible "eye" marks, is also limited in respect to accuracy of detection. The accurate detection of such registration marks is dependent on either the largeness of the mark or, in the case of a small mark, the accuracy with which the detector is registered upon the fluctuating paths in which the marks travel. The accurate detection of traditional eye marks affixed to a plastic or other flexible, stretchable, elastic web requires either; (a) a large eye mark to insure the passage of at least some portion of each mark underneath a stationary detector or, (b) in the case of small eye marks, a sophisticated detector tracking apparatus to insure the consistent registration of the detector upon the fluctuating paths of the moving marks.
Another known approach to maintaining appropriate registration between a web and various work stations is to provide a marginal registration strip with printed or other registration markings. While such an approach can simplify machine set up and registration, as compared with the cyclically blinded detector approach, the strip is trimmed off and becomes scrap so this process is wasteful.
A variation in the technique for controlling the web movement with a removable strip employs gaps or holes positioned along the strip as position indicators for the web. The presence of the gap is detected by a spark-gap detector which completes a circuit by causing a spark to traverse the gap. In this way the presence or absence of gaps or holes along the web is indicated to control circuitry which in turn is used for maneuvering the web
The spark-gap system for web control also has deficiencies. In order to complete a circuit with the use of a spark, it is necessary that a relatively high voltage be maintained between two portions of the spark-gap detector. In some environments, this can be very undesirable. For example, moisture can cause either a malfunction of the spark-gap detector or can provide a path of low electrical resistance which results in a false signal.
A second problem encountered with spark-gap detectors is that the detector cannot tell the difference between intentionally and unintentionally formed gaps or holes. If the control circuitry is activated by the presence of a rip in the registration strip of the web, control functions will be unsynchronized and web material will be wasted.
It has been suggested that magnetization of an area directly on the web with a decorative coating printed over the magnetized area can be used to provide a non-visible control function to the moving web. Magnetized areas are susceptible to detection by various known techniques and have been proposed for providing control coordination. A magnetized area, however, can be affected by its environment in an adverse manner. Electric and magnetic fields in the area of the moving web could create a condition where the detector would not detect the magnetized area and control coordination of movement would be lost. Further, if the magnetized area is placed directly upon the web it is virtually impossible, if not totally so, to hide the magnetized area with a printing overlay and with clear webs the area will be visible from the other side of the web. Thus, a magnetized area detracts from an intended and desired attractive appearance.
Another problem with prior web registration techniques has been that it has been usually necessary to provide some different form of web registration system when the web is used than the system employed in manufacturing the web. For example, if a removable registration strip was employed, that strip is not present when the user is labeling vessels or unloading and sealing bags or envelopes. In commercial machines for loading and sealing AUTOBAG products, spark gap detection has been employed. This has to some extent limited the application of such machines because obviously they cannot be used in explosive or very wet environments. Further, spark-gap detection can present service and other problems.
With the system described and claimed in the previously referenced envelope machine patent for unloading and sealing envelope each envelope is mechanically registered at the load station. While the machine and the system described have enjoyed good commercial success, greater productive capacity than can be achieved with that mechanical registration is desired.
There have been proposals to use visible light detectors in conjunction with materials which absorb ultraviolet light and emit visible light, for registration of work operations. However, until now, there have been no proposals which suggest the use of a wave shift sensitive detector in conjunction with electromagnetiv wave shifting control indicia which emit either visible or invisible electromagnetic radiation for registration of work operations. Neither has anyone suggested the use of non visible electromagnetic wave shifting indicia in a repetitive pattern for control of repetitive work operations on a web.
Perhaps more importantly no one has suggested a web control which both permits complete freedom of choice in web decoration, lack of decoration, and/or the application of informative printing which does not suffer any of the described short comings of "blinded" detectors, hole or gap detection, or a wasted control strip. Thus, there have been no successful proposals for flexible web feed control which are universally useful both in web manufacture and use because all such proposals have had shortcomings such as adversely affecting the appearance of the web. Moreover, even if feasible, little if any use has been made of the same registration techniques for both manufacture and use of a tape or other web, at least with plastic bags, labels and envelopes.